Quantum dots are nano-sized inorganic fluorophores based on semiconductor or metallic materials. They are typically structured as three-dimensional groupings or clusters of atoms (ranging from a few to as many as 10,000) whose electron motion is confined by potential barriers in all three dimensions. This so-called “quantum confinement” phenomenon has significant ramifications for the absorptive and emissive behaviors of the quantum dots. For example, quantum dots exhibit size-dependent fluorescence with narrow emission bandwidths (FWHM˜30 to 45 nm) that span the visible and near infrared (IR) spectra. The quantum dots are further characterized with broad absorption bands, which allow for the simultaneous excitation of several particle sizes (fluorescent colors) at a common wavelength.
Quantum dots are known to exhibit useful spectral characteristics. They have been used as taggants for tracking individual items. Quantum dots, if properly applied to an object, have the advantage that they are invisible under normal lighting conditions, i.e., they do not fluoresce when exposed to the visible range of the light spectrum. When the object marked with the quantum dots is subjected to ultraviolet light the quantum dots fluoresce, providing a bright, easily identifiable image pattern in the visible range (about 400 nm-700 nm). Certain quantum dots fluoresce into the near IR region (about 700 nm-1400 nm) and can be visualized by, for example, night vision goggles.
The fluorescent image patterns can be authenticated or decoded through direct visualization or a reading device under the appropriate light source. This permits specific identification of an object to provide security benefits such as brand protection, and counterfeit prevention. For example, a pattern created on an object by quantum dots can be specifically linked to that product for later tracking and identification. Such a pattern can also be used to prevent counterfeiting of goods and to distinguish an authentic product from a counterfeit product.
Quantum dots have the advantage of being both physically and spectrally undetectable under normal lighting conditions. The microscopic dimensions of the quantum dots allow for their easy incorporation into carrier materials such as solvents, paint, ink, binders or adhesives. These quantum dot formulations can be sprayed, deposited, printed, pressed, or otherwise applied to a substrate of interest.
Currently, when quantum dots are applied to metals such as copper, brass, stainless steel, bronze, aluminum, nickel, etc, the fluorescence intensity of the quantum dots can be dramatically reduced or quenched. In addition, the quantum dots may easily rub off of the substrate materials, particularly smooth materials such as stainless steel and brass. This limits the current use of quantum dots on such metallic surfaces.
Accordingly, there remains a need in the art to provide quantum dot formulations having improved spectral and environmental stability, particularly with respect to metal substrates.